Maybe about 3 feet if you are growing huge monsters. As long as it is deep enough for the roots at the time, it's fine.
Ive heard root mass correlates directly to yield. If thats true, then as thick or (as deep) as you can have it, while being able to maintain it. You can keep a plant small in a small container, you just risk it becoming root bound overtime. This can be trimmed and the plant can continue to live in small conditions. Or as Island said 3ft+ if you want a monster!
Thickness, or depth rather, doesn't matter near as much as total volume. My 15g pot is only about two inches deeper than a 5g, but will grow much larger plants. A 100g smart pot is only five inches taller than the 15g, but will sustain tree sized plants.
Eventually as big as you want... But I would work my way up. If you plant baby seedlings in a 5 gallon pot and saturate the medium, your seedlings' roots will rot because it can't absorb that much water. I would start in a pot the size of a solo cup....or an actual solo cup, then to a 1 gallon, then I'd say your free to do whatever you want.
My last one was in a 3gal pot, and when i chopped her, the roots OBVIOUSLY needed more room. Going with 5gal buckets this time around. Sent from my DROID4 using Grasscity Forum mobile app